1. Field of the Invention:
This invention relates in general to the field of proximity switches for sensing the position of a piston within a fluid operated, expansible chamber cylinder, and more specifically to the field of a mounting device which provides a secure, vibration-resistant mounting of the proximity switch to the cylinder.
2. Description of the Prior Art:
In the use of fluid operated, expansible chamber cylinders, it has been common to use magnetic switches for sensing the position of the piston within the cylinder. The magnetic switch is typically a magnetic reed switch and opens and closes in response to a magnetic flux produced by a permanent magnet attached to a piston within the cylinder. Movement of the piston and the magnet carried thereon varies the magnetic flux and opens and closes the associated reed switch. Proximity switches of this type are very widely used for such applications as automatic cylinder cycling, light indication, cylinder programming and sequencing, multi-position signalling, machining applications, etc. Their use permits the elimination of a large number of mechanical elements formally associated with the use of mechanical limit switches, such as auxiliary gears, spiral rod extensions, switch dogs, mounting plates and cams.
Conventional proximity switches, such as those commercially available under the name Parker Fluidpower, are mounted on a tie-rod extending between the cylinder heads secured to each end of the cylinder. The proximity switch is provided with a J-shaped channel formed on one side thereof which is engageable with the tie-rod. A plurality of threaded bores, typically two, are formed in the proximity switch and extend through the longer side of the J-shaped channel. Externally threaded fastening devices are provided which are engageable with the threaded bores. These fastening devices are, typically, bolts and nuts. The proximity switch is mounted to the cylinder by engaging the tie-rod in the channel and tightening down the fastening devices until they abut tightly against the tie-rod.
Due to the manner of mounting the proximity switch, the device is subject to several types of failure in operation. Since the piston typically operates at high speeds, considerable vibration occurs in the body of the cylinder. The vibration tends to loosen the fastening devices, thus causing the proximity switch to slip and rotate around the tie-rod. Because of this rotation, the proximity switch is no longer positioned against a surface of the cylinder. An air gap is formed between the switch and the magnet disposed on the piston inside the cylinder and diminishes the magnitude of the magnetic flux experienced by the switch as the magnet passes by. This greatly diminishes the performance of the proximity switch.
In addition, since the proximity switch carries a number of electrical leads extending therefrom, it may easily be dislodged by accidental pulling or jerking of the leads. The force exerted on the leads will cause the switch to pivot around the tie rod into a dislodged position.
Several solutions have been proposed to this problem of slippage of the proximity switch, but all have proven unsatisfactory or impractical. Serrations have been provided to better grip the tie-rod. U.S. Pat. No. 4,086,456 proposes providing a tie-rod of hexagonal cross-section, instead of the typical circular cross-section. The proximity switch is provided with a first gripping surface which engages the tie-rod to position the switch with respect to the cylinder. A fastening plate is provided which has a second gripping surface for engaging the tie-rod and positioning the proximity switch. The plate is then tightened against the proximity switch by means of a fastening device. One of the surfaces of the proximity switch is made arcuate in shape to securely engage the surface of the cylinder.
While the above-referenced patent may be successful in providing a secure, vibration resistent mounting of the proximity switch to the cylinder, the device disclosed therein is not usable with the standard type of proximity switch and the standard type of tie-rod provided on a fluid cylinder. Thus, the problem of slippage with the standard type of proximity switch continues to exist.
It would be desirable to provide a simple means of securely mounting a proximity switch to a fluid cylinder which will resist dislodgement of the proximity switch due to vibration.
It would also be desirable to provide such a secure, vibration resistant mounting which may be used with a standard type of proximity switch and a fluid cylinder provided with a standard, circular tie-rod.